1. Field of the Invention
The present invention relates to aging of a grating built in an optical waveguide, and more particularly to aging of a grating used as a filter, multi/demultiplexer, dispersion-compensator, and the like in an optical fiber network. The present invention also relates to a temperature sensor including an optical waveguide grating as a sensing section.
2. Related Background Art
An optical waveguide type grating, which is typified by an optical fiber grating, is a region in an optical waveguide such as an optical fiber (mostly in its core portion) in which a periodic change of refractive index along the longitudinal direction of the waveguide occurs. The region where the refractive index changes can transmit or reflect propagated light depending on its wavelength. In particular, a Bragg grating generates reflected light with a narrow wavelength band centered on its Bragg wavelength. The optical waveguide grating is applied to various optical elements such as filters, multi/demultiplexers, dispersion-compensators, and the like.
FIG. 1 is a view showing a typical method for producing an optical waveguide grating. As shown in FIG. 1, a grating 20 is often formed by a method comprising preparing a silica-based optical fiber 10 in which GeO.sub.2 (germanium dioxide) is added to at least its core region; irradiating this optical fiber 10 with an interference fringe formed by light rays 30 having a predetermined wavelength; and generating a change in refractive index independent on the optical energy intensity distribution of this interference fringe. Since the optical fiber 10 is usually coated with a plastic layer (not shown), a part of the coating is removed, and thus exposed part of the optical fiber 10 is irradiated with the light rays 30. It has been considered that the irradiation with a certain wavelength of light generates Ge-defects in the GeO.sub.x -doped portion in the silica-based optical waveguide, thereby causing the change in refractive index. In FIG. 1, numeral 22 indicates parts where a larger amount of increase in refractive index is induced upon the irradiation, whereas numeral 24 indicates parts where a smaller amount of increase in refractive index is induced. The grating 20 may be considered to be a region where the parts 22 and 24 are alternatively and periodically disposed along the longitudinal direction of the optical fiber 10.
An optical waveguide grating may be used as a temperature sensor also. The temperature sensor comprises an optical waveguide grating as a sensing section, and measures temperature utilizing the temperature dependence of the Bragg wavelength. More particularly, in the measurement of temperature, the sensor measures the Bragg wavelength and compares the measured value with the temperature dependence of the Bragg wavelength previously measured to determine the temperature.
As is previously known, the characteristics of an optical waveguide grating change over time because the number of Ge-defects generated by the irradiation of light changes over time. This has been known as aged deterioration of an optical waveguide grating. With respect to a Bragg grating, the Bragg wavelength at any temperature changes (usually decreases) over time. It means that the operating characteristics of a temperature sensor comprising an optical waveguide Bragg grating as its sensing section change over time. For example, if such a temporal change is relatively rapid, different Bragg wavelengths are measured for the same temperature one month and three months after beginning to use the temperature sensor, and thus different temperatures will be determined at different points in time. In view of the foregoing, there have been proposed techniques which perform accelerated aging for an optical waveguide grating immediately after its manufacture to sufficiently suppress its aged deterioration upon operation in the market. Examples of such techniques are disclosed in U.S. Pat. Nos. 5,287,427 and 5,620,496 which are incorporated herein by reference.
In the technique disclosed in U.S. Pat. No. 5,620,496, normalized refractive index difference .eta. is supposed to be represented by the following relational expression: ##EQU1##
where t represents time, and C and .alpha. are functions of temperature. The normalized refractive index difference .eta. is a value of the refractive index difference of a grating when time t has elapsed from a predetermined point of time (i.e., reference time) after formation of the grating, and this value is normalized with respect to the refractive index difference of the grating at this point of time. Namely, .eta.=(refractive index difference at t after the reference time)/(refractive index difference at the reference time). In the technique disclosed in the above patent, the time immediately after formation of a grating is adopted as the reference time. The refractive index difference refers to the difference between the maximum and minimum values of the refractive index in a grating.
In the conventional techniques, from the fact that .eta. changes more rapidly as temperature is higher, the optical fiber grating is heat-treated in an environment with temperature higher than its operating temperature to perform the accelerated aging, in order to suppress the deterioration upon its operation.